The incidence of human immunodeficiency virus (HIV) and several other sexually transmitted infections (STI) is disproportionately high among African-American women as compared to women of other racial/ethnic groups (CDC, 2005a). Most infections in women are attributed to sexual contact with men (CDC, 2005b). Traditional approaches to sexual risk reduction have focused on individual behavior. However, some of the sexual relationship practices that increase women's sexual exposures may be related to characteristics of the social environment. Qualitative data suggest that some African-American women perceive that the relative lack of available heterosexual partners increases heterosexual risk behavior (Sobo 1993, 1995; Thomas & Thomas, 1999; Adimora & Shoenbach 2005). Few studies have considered the quantitative relationship between the relative availability of heterosexual partners or other environmental characteristics with sexual behavior or HIV/STI incidence or prevalence. The proposed analysis addresses the lack of information regarding the social context of racial/ethnic disparities in the prevalence of HIV/AIDS and STI in women in the U.S. The relative availability of heterosexual partners is reflected in the male-female sex ratio among nonincarcerated adults. The sex ratio and other characteristics of the social environment related to economic resources will be obtained from the U.S. Census Bureau and other sources. Individual data regarding sexual behavior, and self reports and biological markers of infection with HIV/STI will be obtained from the National Health and Nutrition Examination Survey (NHANES). The data will be linked geographically and analyzed using weighted multilevel regression modeling. The application of multilevel modeling constitutes an innovation in the analysis of the effects of environmental factors on sexual behavior and HIV/STI. Racial/ethnic disparities in HIV/STI infection rates among women may be understood more completely with a consideration of the sex ratio and other features of the social context of sexual relationship practices This research is relevant to public health because an imbalance in the number of men and women or other characteristics of the social environment may be related to increased rates of HIV and other sexually transmitted infections. Determining whether this is the case is important to design the best ways to prevent these illnesses. Lowering the highest rates of these illnesses can help to increase health and life expectancy overall, and help to reduce health disparities among racial/ethnic groups. [unreadable] [unreadable] [unreadable]